Working as a home inspector in Bellingham, WA, the heart of the Pacific NorthWET, I see many circumstances that are conducive to decay and deterioration of the building materials.
This example, below, is commonly seen. A homeowner has tacked indoor-outdoor carpet over the wood deck. Some of the surface, of lesser concern, is covered with an overhang above. But there is a significant expanse that is not covered. These carpets absorb moisture and retain it against, and in, the surface of the lumber below.
A combination of wet wood and carpet over the wood is a double whammy. With the arrangement, nature gets involved -- wood that is at 20% or higher moisture levels is susceptible to rot. And, at least in this wet maritime climate, the question is not"whether" the wood will rot, the question is simply "when" will rot occur in this wood that stays wet almost all of the time.
It is my suggestion, when building decks, that structural components such as posts, joists, beams and ledgers (where possible) should consist of pressure-treated lumber. And the decking, especially wood boards, should be installed with rain gaps or spaces between the planks -- so rain does not collect on the surface. Of course, carpeting should not be tacked over the wood, doing so defeats the purpose of the rain gaps and leads to rot.